[css-color-4] Add a98-rgb diagram

Author: Chris Lilley

css-color-4/Overview.bs

@@ -2299,6 +2299,18 @@ Predefined colorspaces: ''srgb'', ''display-p3'', ''a98-rgb'', ''prophoto-rgb'',
 				<tr><th>Black luminance</th><td colspan="2">0.5557 cd/m<sup>2</sup></td></tr>
 			</table>
 
+			<figure>
+				<img src="images/a98-prim-sec.svg" style="width: 60vmin"
+				alt="diagram of a98 primaries and secondaries in LCH">
+				<figcaption>Visualization of the a98 colorspace in LCH.
+					The primaries and secondaries are shown
+					(but in sRGB, not in the correct colors).
+					For comparison, the sRGB primaries and secondaries
+					are also shown, as dashed circles.
+					P3 primaries have higher Chroma, especially the green.
+				</figcaption>
+			</figure>
+
 		<dt><dfn>prophoto-rgb</dfn>
 			<dd>
 				The ''prophoto-rgb'' colorspace accepts three numeric parameters,